Showing papers in "IEEE\/ASME Journal of Microelectromechanical Systems in 2015"

The Piezoresistive Effect of SiC for MEMS Sensors at High Temperatures: A Review

Abstract: Silicon carbide (SiC) is one of the most promising materials for applications in harsh environments thanks to its excellent electrical, mechanical, and chemical properties. The piezoresistive effect of SiC has recently attracted a great deal of interest for sensing devices in hostile conditions. This paper reviews the piezoresistive effect of SiC for mechanical sensors used at elevated temperatures. We present experimental results of the gauge factors obtained for various poly-types of SiC films and SiC nanowires, the related theoretical analysis, and an overview on the development of SiC piezoresistive transducers. The review also discusses the current issues and the potential applications of the piezoresistive effect in SiC. [2015-0092]

Modeling, Fabrication, and Characterization of Piezoelectric Micromachined Ultrasonic Transducer Arrays Based on Cavity SOI Wafers

Abstract: This paper presents high fill-factor piezoelectric micromachined ultrasonic transducer (PMUT) arrays fabricated via a novel process using cavity SOI wafers. The simple three-mask fabrication process enables smaller diameter PMUTs (25 $\mu $ m) and finer pitch than previous processes requiring through-wafer etching. PMUTs were fabricated with diameters from 25 to 50 $\mu $ m, resulting in center frequencies from 13 to 55 MHz in air. Two types of devices, having different piezoelectric layers, lead zirconium titanate (PZT), and aluminum nitride (AlN), were fabricated and characterized. Comparing 50- $\mu $ m diameter devices, the PZT PMUTs show large dynamic displacement sensitivity of 316 nm/V at 11 MHz in air, which is $\sim 20\times $ higher than that of the AlN PMUTs. Electrical impedance measurements of the PZT PMUTs show high electromechanical coupling ${k} _{t}^{2}=12.5$ % and 50- $\Omega $ electrical impedance that is well-matched to typical interface circuits. Immersion tests were conducted on PZT PMUT arrays. The fluid-immersed acoustic pressure generated by an unfocused $9\times 9$ array of 40- $\mu $ m diameter, 10-MHz PZT PMUTs, measured with a needle hydrophone 1.2 mm away from the array, was 58 kPa with a 25 $V_{\mathrm {pp}}$ input. Beam forming based on electronic phase control produced a narrow, 150- $\mu $ m diameter, focused beam over a depth of focus >0.2 mm and increased the pressure to 450 kPa with 18 $V_{\mathrm {pp}}$ input. [2014-0324]

Flexible Capacitive Tactile Sensor Array With Truncated Pyramids as Dielectric Layer for Three-Axis Force Measurement

Abstract: This paper presents a flexible capacitive tactile sensor array embedded with a truncated polydimethylsiloxane pyramid array as a dielectric layer. The proposed sensor array has been fabricated with $4\times 4$ sensor units. The measurement ranges of forces in the $x$ -axis, $y$ -axis, and $z$ -axis are 0–0.5, 0–0.5, and 0–4 N, respectively. In the range of 0–0.5 N, the sensitivities of the sensor unit are 58.3%/N, 57.4%/N, and 67.2%/N in the $x$ -axis, $y$ -axis, and $z$ -axis, respectively. In the range of 0.5–4 N, the sensitivity in the $z$ -axis is 7.7%/N. Three-axis force measurement has been conducted for all the sensor units. The average errors between the applied and calculated forces are 11.8% ± 6.4%. The sensor array has been mounted on a prosthetic hand. A paper cup and a cube are grasped by the prosthetic hand and the three-axis contact force is measured in real time by the sensor array. Results show that the sensor can capture the three-axis contact force image both in light and tight grasping. The proposed capacitive tactile sensor array can be utilized in robotics and prosthetic hand applications. [2014-0350]

Anchor Losses in AlN Contour Mode Resonators

Abstract: In this paper, we analyze possible sources of dissipation in aluminium nitride (AlN) contour mode resonators for three different resonance frequency devices ( ${f} _{\!r}$ ) (220 MHz, 370 MHz, and 1.05 GHz). For this purpose, anchors of different widths ( ${W} _{\!a}$ ) and lengths ( ${L} _{\!a}$ ) proportional to the acoustic wavelength ( $\lambda $ ) are designed as supports for resonators in which the dimensions of the vibrating body are kept fixed. The ${Q}$ extracted experimentally confirms that anchor losses are the dominant source of damping for most anchor designs when ${f} _{\!r}$ is equal to 220 and 370 MHz. For specific anchor dimensions ( ${W} _{\!a}$ / $\lambda $ is in the range of 1/4–1/2) that mitigate energy leakage through the supports, a temperature-dependent dissipation mechanism dominates as seen in higher ${f} _{\!r}$ resonators operating close to 1.05 GHz. To describe the ${Q}$ due to anchor losses, we use a finite-element method with absorbing boundary conditions. We also propose a simple analytical formulation for describing the dependence of the temperature-dependent damping mechanism on frequency. In this way, we are able to quantitatively predict ${Q}$ due to anchor losses and qualitatively describe the trends observed experimentally. [2014-0232]

Temperature Dependence of the Elastic Constants of Doped Silicon

Abstract: Resonators fabricated in heavily doped silicon have been noted to have a reduced frequency-temperature dependence compared with lightly doped silicon. The resonant frequency of silicon microelectromechanical systems (MEMS) resonators is largely governed by the material’s elastic properties, which are known to depend on doping. In this paper, a suite of different types and orientations of resonators were used to extract the first- and second-order temperature dependences of the elastic constants of p-doped silicon up to 1.7e20 $\mathrm{cm}^{\mathrm {\mathbf {-3}}}$ , and n-doped up to 6.6e19 $\mathrm{cm}^{\mathrm {\mathbf {-3}}}$ . It is shown that these temperature-dependent elastic constants may be used in finite element analysis to predict the frequency-temperature dependence of similarly doped silicon resonators. [2013-0331]

Demonstration of 1 million Q -factor on microglassblown wineglass resonators with out-of-plane electrostatic transduction

Abstract: In this paper, we report Q-factor over 1 million on both n = 2 wineglass modes, and high-frequency symmetry (Af/f ) of 132 ppm on wafer-level microglassblown 3-D fused silica wineglass resonators at a compact size of 7-mm diameter and center frequency of 105 kHz. In addition, we demonstrate for the first time, out-of-plane capacitive transduction on microelectromechanical systems wineglass resonators. High Q-factor is enabled by a high aspect ratio, self-aligned glassblown stem structure, careful surface treatment of the perimeter area, and low internal loss fused silica material. Electrostatic transduction is enabled by detecting the spatial deformation of the 3-D wineglass structure using a new out-of-plane electrode architecture. Out-of-plane electrode architecture enables the use of sacrificial layers to define the capacitive gaps and 10 μm capacitive gaps have been demonstrated on a 7-mm shell, resulting in over 9 pF of active capacitance within the device. Microglassblowing may enable batch-fabrication of high-performance fused silica wineglass gyroscopes at a significantly lower cost than their precision-machined macroscale counterparts.

Mode-Matching of Wineglass Mode Disk Resonator Gyroscope in (100) Single Crystal Silicon

Abstract: In this paper, we present four design methods to overcome (100) silicon crystalline anisotropy and achieve mode-matching in wineglass-mode disk resonator gyroscope (DRG). These methods were validated through experimental characterization of more than 145 different devices that arose from simulations. With the proposed methods, the frequency split of the 250-kHz DRG wineglass modes in (100) silicon was reduced from >10 kHz to as low as 96 Hz (<0.04% of 250-kHz resonant frequency) without any electrostatic tuning. Perfect mode-matching is then achieved using electrostatic tuning. Mode-matching was maintained within ±10 Hz over a temperature range from −20 °C to 80 °C. The temperature dependence of quality factor is also discussed in this paper. These results allow for the development of high-performance miniature DRGs tuned for degenerate wineglass mode operation from high-quality crystalline silicon material. [2013-0303]

A High Fill-Factor Annular Array of High Frequency Piezoelectric Micromachined Ultrasonic Transducers

Abstract: This paper presents a 1.2-mm diameter high fill-factor array of 1261 piezoelectric micromachined ultrasonic transducers (PMUTs) operating at 18.6 MHz in fluid for intravascular ultrasound imaging. At 1061 transducers/ $\mathrm{mm}^{2}$ , the PMUT array has a 10–20 times higher density than previous PMUT arrays realized to date. Aluminum nitride (AlN)-based PMUTs described in this paper are fabricated using a process compatible with the fabrication of inertial sensors, radio frequency (RF) resonators, and CMOS integrated circuits. The PMUTs are released using a front-side sacrificial etch through etching holes that are subsequently sealed by a thin layer of parylene. Finite element method and analytical results, including resonant frequency, pressure sensitivity, output acoustic pressure, and directivity are given to guide the PMUT design effectively, and are shown to match well with measurement results. Due to the PMUTs thin membrane (750-nm AlN/800-nm SiO2) and small diameter, a single 25- $\mu $ m PMUT has approximately omnidirectional directivity and no near-field zone with irregular pressure pattern. PMUTs are characterized in both the frequency and time domains. Measurement results show a large displacement response of 2.5 nm/V at resonance and good frequency matching in air, high center frequency of 18.6 MHz and wide bandwidth of 4.9 MHz, when immersed in fluid. Phased array simulations based on measured PMUT parameters show a tightly focused high-output pressure acoustic beam. [2014-0114]

3-D Integration and Through-Silicon Vias in MEMS and Microsensors

Abstract: After two decades of intensive development, 3-D integration has proven invaluable for allowing integrated circuits to adhere to Moore’s Law without needing to continuously shrink feature sizes The 3-D integration is also an enabling technology for hetero-integration of microelectromechanical systems (MEMS)/microsensors with different technologies, such as CMOS and optoelectronics This 3-D hetero-integration allows for the development of highly integrated multifunctional microsystems with small footprints, low cost, and high performance demanded by emerging applications This paper reviews the following aspects of the MEMS/microsensor-centered 3-D integration: fabrication technologies and processes, processing considerations and strategies for 3-D integration, integrated device configurations and wafer-level packaging, and applications and commercial MEMS/microsensor products using 3-D integration technologies Of particular interest throughout this paper is the hetero-integration of the MEMS and CMOS technologies [2015-0158]

High Resolution Magnetometer Based on a High Frequency Magnetoelectric MEMS-CMOS Oscillator

Abstract: This paper demonstrates a miniaturized and high resolution (16 nT/Hz 1/2 ) magnetometer based on a high frequency (168.1 MHz) magnetoelectric Microelectromechanical Systems-Complementary metal-oxidesemiconductor (MEMSCMOS) oscillator. For the first time, a high frequency and high electromechanical performance (quality factor, Q ~ 1084 and electromechanical coupling coefficient, k t 2 ~ 1.18%) magnetoelectric micromechanical resonator based on a self-biased aluminum nitride/iron-gallium-boron (AlN/FeGaB) bilayer nanoplate (250/250 nm) is implemented and used to synthesize a low noise frequency source (2.7 Hz/Hz 1/2 ) whose output frequency is highly sensitive to external magnetic field (169 Hz/μT at zero magnetic field bias). The angular sensitivity of the magnetometer for electronic compass applications is also investigated showing an ultrahigh angular resolution of 0.34° for a 10-μT conservative estimate of the earth's magnetic field, due to the strongly anisotropic sensitivity of the self-biased AlN/FeGaB magnetoelectric resonator. This paper represents the first demonstration of a high resolution self-biased MEMS magnetoelectric resonant sensor interfaced to a compact and low power self-sustained CMOS oscillator as direct frequency readout for the implementation of miniaturized and low power magnetometers with detection limit pushed in ~10s nT/Hz 1/2 range.

Development of a Broadband Triboelectric Energy Harvester With SU-8 Micropillars

Abstract: This paper describes a broadband energy harvester working on the principle of contact electrification or triboelectric charging. Design and fabrication of the device have been dis- cussed. The device uses contact and separation mechanism using a cantilever to generate triboelectric charges. This mechanism introduces nonlinearity in the cantilever, which results in broad- band behavior of triboelectric energy harvester. The device uses SU-8 micropillar arrays to enhance the triboelectric charging. A study is conducted to study the effect of the micropillar sizes on the power output of devices. The devices were tested at different acceleration levels. The peak power output achieved is 0.91 µ Wa t an acceleration of 1g. The amplitude limiter based design of the energy harvester enables broadening of operating bandwidth as the acceleration level increases. A maximum operating bandwidth of 22.05 Hz was observed at 1.4g increasing from an operating bandwidth of 9.43 Hz at 0.4g. (2013-0401)

Silk-backed structural optimization of high-density flexible intracortical neural probes

Abstract: Many chronic neuroscience studies require neural probes that can reliably record with a large number of electrodes in a densely configured array. Previous works have shown that adverse tissue reaction can be significantly reduced as probe shanks are scaled down toward subcellular dimensions. In addition, flexible probes can mitigate shear stress-induced tissue damage due to micromotion. However, both size reduction and flexibility compromise probe's ability to penetrate the pia mater, especially when many electrodes are distributed across multiple probe shanks. In this paper, we present a method to lithographically pattern a biodegradable silk coating that provides temporary mechanical stiffness for the surgical insertion of flexible probes without any conventional design constraints on the probe size, shape, or material. After insertion, the silk is completely dissolved in the tissue, only leaving the flexible minimum-geometry probes inside the brain. We validated the design by successfully inserting silk-backed 64-channel parylene probes into the motor cortex of Long-Evans rats (n = 6) and recorded in vivo neural activity for six weeks.

Manipulating Liquid Metal Droplets in Microfluidic Channels With Minimized Skin Residues Toward Tunable RF Applications

Abstract: A nontoxic liquid metal, such as eutectic gallium-indium (EGaIn) alloy, has been used to develop tunable radio frequency (RF) components, such as antennas, inductors, or capacitors, for enabling large tunable range, better linearity, and low loss, using fluidic displacement of the liquid metal. However, EGaIn residue, due to its fast oxidation, limits multiple movement of the EGaIn in the reconfigurable RF components. This paper focuses on the use of surfactants, carrier liquids, and microchannel coating materials that minimize EGaIn fragmentation and EGaIn residues on poly(dimethylsiloxane) (PDMS)-based microfluidic channels during repeated actuation of an EGaIn plug. Using a combination of carrier liquids and microchannel coating materials to minimize EGaIn from leaving residues on the PDMS microfluidic channel, a microstrip transmission line switch as a proof-of-concept reconfigurable RF application using the EGaIn plug is demonstrated. It is switched ON<4 dB and OFF with a loss of <18 dB over the frequency range between 4 and 15 GHz. [2014-0228]

A Monolithic CMOS-MEMS Oscillator Based on an Ultra-Low-Power Ovenized Micromechanical Resonator

Abstract: A fully monolithic complimentary metal–oxide– semiconductor-microelectormechanical systems (CMOS-MEMS) oscillator comprised of an ovenized double-ended tuning fork resonator to enable ultra-low heater power operation of only 0.47 mW over entire temperature span (–40 °C to 85 °C) and a low noise sustaining circuit to achieve low phase noise has been demonstrated in a Taiwan Semiconductor Manufacturing Company (TSMC) 0.35- $\mu $ m CMOS process. The combination of low thermal conductivity material and high thermal isolation design is the key to attaining ultra-low-power heater operation in a sub-mW level. Passive temperature compensation scheme is also conducted in the proposed CMOS-MEMS resonator by an oxide-metal composite structure, showing a low temperature coefficient of frequency (TC $_{f}$ ) of only +5.1 ppm/°C, which is suited for the use in ovenized oscillator systems. By implementing a constant-resistance temperature control scheme, the frequency drift of the resonator smaller than 120 ppm from −40 °C to 85 °C is demonstrated in this paper, indicating an equivalent TC $_{f}$ smaller than 1 ppm/°C, a record-low value against its CMOS-MEMS counterparts. The CMOS-MEMS oscillator operating at 1.2 MHz demonstrates a phase noise of −112 dBc/Hz at 1-kHz offset and −120 dBc/Hz at 1-MHz offset while drawing less than 1.3 mW. The entire power consumption of the ovenized oscillator system is confirmed to be less than 1.8 mW (oscillator + micro-oven), verifying the great potential of low power oven-controlled MEMS oscillators realized in CMOS-MEMS technology. [2014-0068]

Zero-Bending Piezoelectric Micromachined Ultrasonic Transducer (pMUT) With Enhanced Transmitting Performance

Abstract: A piezoelectric micromachined ultrasonic transducer (pMUT) has enabled numerous exciting ultrasonic applications. However, residual stress and initial buckling may worsen the transmitting sensitivity of a pMUT, and also limit its application and commercialization. In this paper, we report a new innovative pMUT with a perfectly flat membrane, i.e., zero-bending membrane. Leveraging on the stress-free AlN thin film, framelike top electrode layout, and integrated vacuum cavity, the initial deflection of suspended membrane is significantly suppressed to only 0.005%. The transmitting sensitivity of the zero-bending pMUT is measured as 123 nm/V at a resonant frequency of 2.21 MHz, which is 450% higher than that of the reference pMUT with slightly non-zero initial deflection. Compared with the simulation results, the measured data of zero-bending pMUT achieve 94.5% of its ideal transmitting sensitivity. It is solid evidence that our approach is an effective and reliable way to overcome the residual stress and the initial buckling issue. [2015-0093]

Design and Fabrication of S 0 Lamb-Wave Thin-Film Lithium Niobate Micromechanical Resonators

Abstract: Commercial markets desire integrated multifrequency band-select duplexer and diplexer filters with a wide fractional bandwidth and steep roll-off to satisfy the ever-increasing demand for spectrum. In this paper, we discuss the fabrication and design of lithium niobate (LN) thin-film S0 Lamb-wave resonators on a piezoelectric-on-piezoelectric platform. Filters using these resonators have the potential to fulfill all the above requirements. In particular, we demonstrated one-port high-order S0 Lamb-wave resonators with resonant frequencies from $\sim 400$ MHz to $\sim 1$ GHz on a black rotated y-136 cut LN thin film. The effective electromechanical coupling factor ( $k_{eff}^{2}$ ) ranges from 7% to 12%, while the measured quality factor ranges from 600 to 3300. The highest $k_{eff}^{2} \times Q$ achieved on this chip is 194, significantly surpassing contour mode resonators manufactured in other technologies. [2014-0280]

Electromagnetic Energy Harvester With Flexible Coils and Magnetic Spring for 1–10 Hz Resonance

Abstract: This paper presents an energy harvester with microfabricated flexible coils (rolled and aligned to a magnet array for maximum magnetic flux change) and magnetic spring to generate electrical power from human body motion. The magnet array is suspended by a magnetic spring for a resonant frequency of several hertz. An analytical model of vibration-driven energy harvester with magnetic spring through magnet and coil arrays is developed to explore the power generation with various magnet ranges and vibration amplitudes. Experimental results show that the electromagnetic energy harvester with six 7-turn microfabricated coils (occupying 3.8 cc and weighing 8.5 gram) generates an induced electromotive force (EMF) of $\mathrm{V}_{\mathrm {rms}} =6.7$ mV with 0.53- $\mu $ W power output (into 21- $\Omega $ load) from 0.27-g acceleration at 8 Hz (corresponding to 1.05-mm vibration amplitude). Its larger-scale version with sixteen 200-turn wire-wound coils (occupying 26 cc and weighing 98 gram) generates an EMF of $\mathrm{V}_{\mathrm {rms}} =1.3$ V with 4.3-mW power output (into 100- $\Omega $ load) from 0.5-g acceleration at 5.5 Hz (corresponding to 4.1-mm vibration amplitude). When the larger-scale version of the energy harvester is placed in a backpack of a human walking at various speeds, the power output is increased as the walking speed is increased from 0.45 m/s (slow walking) to 2.68 m/s (slow running), and reaches 14.8 mW at 2.68 m/s. [2014-0323]

A MEMS Low-Noise Sound Pressure Gradient Microphone With Capacitive Sensing

Abstract: A silicon microelectromechanical systems microphone is described that detects sound pressure gradients. The diaphragm consists of a stiffened plate that rotates around a central axis in response to sound pressure gradients. The motion of the diaphragm is converted into an electronic signal through the use of interdigitated comb fins that enable capacitive sensing. Measured results show that the microphone achieves a substantially lower low-frequency sound pressure-referred noise floor than can be achieved using existing dual miniature microphone systems. Measured directivity patterns are shown to be very close to what is expected for sound pressure gradient receivers over a broad range of frequencies.

Accurate Modeling of Quality Factor Behavior of Complex Silicon MEMS Resonators

Abstract: The quality factor of a resonator represents the decay of vibrational energy over time, and is directly related to the frequency response and other key parameters that determine performance of inertial sensors and oscillators. Accurate prediction of the quality factor is essential for designing high-performance microelectromechanical (MEMS) devices. Several energy dissipation mechanisms contribute to the quality factor. Due to computational complexity, highly simplified models for the dominant dissipation mechanism, such as Zener’s model for thermoelastic dissipation (TED), are often employed. However, the intuition provided by these models is inadequate to predict the quality factor of more complex designs and can be highly misleading. In this paper, we construct complete, quantitative, and predictive models with finite-element methods for the intrinsic energy dissipation mechanisms in MEMS resonators using full anisotropic representation of crystalline silicon and the temperature dependence of all parameters. We find that TED is often a more significant source of damping than has been assumed, because of the previously neglected role of crystalline anisotropy and small geometric features, such as etch release holes—all of which can now be included in practical models. We show that these models, along with simpler scaling models for extrinsic dissipation mechanisms, explain measurements of quality factor in diverse sets of MEMS resonators with unprecedented accuracy. [2014-0106]

MEMS Scale PVDF-TrFE-Based Piezoelectric Energy Harvesters

Abstract: This paper presents the design, the fabrication, and the performance characterization of microelectromechanical systems (MEMS) scale cantilever-type piezoelectric energy harvesters (PEHs) that utilize the piezoelectric polymer polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE). Ranges are determined for device dimensions according to the calculations based on mathematical models. Designed devices were fabricated using standard MEMS fabrication techniques. Electrodes were formed with sputtered Al and Ti/Al thin films, and a 1.3- $\mu \text{m}$ -thick PVDF-TrFE film was deposited using spin coating. Cantilevers were suspended using a two-step process: backside DRIE to perform the bulk etch, followed by XeF2 gaseous etch for the final release. Remnant polarization and coercive field of the fabricated devices were measured as $6.1~\mu \text{C}/\text{cm}^2$ and 74.9 V/ $\mu \text{m}$ , respectively. Piezoelectric performances were evaluated by a press-and-release type of measurement. For these measurements, custom-made probe tips attached to micropositioners were used. Based on the experimental results, maximum power output was calculated as 35.1 pW for a peak tip displacement of $500~\mu \text{m}$ from a $1200~\mu \text {m} \times 300 ~\mu \text{m}$ cantilever, which corresponds to a power output density of 97.5 pW/mm2. The proposed method has the potential to create the PEHs that are monolithically integrated with complementary metal-oxide–semiconductor circuits and lead to self-sustained low power electronics. [2015-0117]

A Bulk-Micromachined Three-Axis Capacitive MEMS Accelerometer on a Single Die

Abstract: This paper presents a high-performance three-axis capacitive microelectromechanical system (MEMS) accelerometer implemented by fabricating individual lateral and vertical differential accelerometers in the same die The fabrication process is based on the formation of a glass-silicon-glass multi-stack First, a 35- $\mu \text{m}$ thick $\langle 111\rangle $ silicon structural layer of an Silicon-On-Insulator (SOI) wafer is patterned with deep reactive ion etching (DRIE) and attached on a base glass substrate with anodic bonding, whose handle layer is later removed Next, the second glass wafer is placed on the top of the structure not only for allowing to implement a top electrode for the vertical accelerometer, but also for acting as an inherent cap for the entire structure The fabricated three-axis MEMS capacitive accelerometer die measures $12\,\times \,7\,\times \,1$ mm3 The $x$ -axis and $y$ -axis accelerometers demonstrate measured noise floors and bias instabilities equal to or better than 55 $\mu \text{g}/\surd $ Hz and 22 $\mu \text{g}$ , respectively, while the $z$ -axis accelerometer demonstrates $126~\mu \text{g}/\surd $ Hz noise floor and $174~\mu \text{g}$ bias instability values using hybrid-connected fourth-order sigma–delta CMOS application specific integrated circuit (ASIC) chips These low noise performances are achieved with a measurement range of over ±10 g for the $x$ -axis and $y$ -axis accelerometers and +12/−75 g for the $z$ -axis accelerometer, suggesting their potential use in navigation grade applications [2014-0351]

Simultaneous Remote Sensing of Temperature and Humidity by LC-Type Passive Wireless Sensors

Abstract: This paper presents an integrated wireless passive sensor for remotely monitoring both temperature and relative humidity. The sensor consists of a coil and a capacitor to form an inductor–capacitor ( $LC$ ) resonant circuit, which oscillates electrically at its resonant frequency. The inductor was a single-layer planar spiral copper inductor and the capacitor was fabricated by the silicon-on-glass process, which utilizes graphene oxide films as the sensing material. The change of the capacitance due to environmental humidity variation shifts the resonant frequency, while environmental temperature affects the resistance and capacitance of the $LC$ circuit and changes the resonant frequency and quality factor. By monitoring the real portion magnitude maximum of the impedance and the resonant frequency for the sensor, it is possible to get the capacitance and resistance from which the temperature and humidity can be extracted. The results presented here show that the sensitivity of the passive wireless sensor is about −17.80 kHz/%RH and $7.32~\Omega $ /%RH at 25 °C from 55%RH to 95%RH, and it is about −7.69 kHz/°C and $6.27~\Omega $ /°C at 65%RH from 10 °C to 40 °C. [2014-0311]

Harvesting Energy From a Rotating Gear Using an AFM-Like MEMS Piezoelectric Frequency Up-Converting Energy Harvester

Abstract: This paper presents an analytical and experimental study of a compact configuration to harvest energy from a rotating gear using piezoelectric microelectromechanical system harvesters. The reported configuration realizes a contact-type frequency up-conversion mechanism in order to generate useful electrical energy. The up-conversion mechanism was achieved using an atomic force microscope (AFM)-like piezoelectric cantilever plucked by the teeth of the rotating gear that could be eventually driven by an oscillating mass. This paper describes relevant design guidelines for harvesting energy from the low-frequency mechanical movement of a rotating gear through analytical modeling and finite element method (FEM) simulation followed by experimental validation. Different harvester configurations are investigated to identify the optimal configuration in terms of the output energy and energy conversion efficiency. The latter results are reported for the first time because of the implementation of an original concept based on the coupling of the harvester with a rotational flywheel. The experimental results reveal that free vibrations of the harvester after plucking contribute significantly to the output energy and efficiency. By adding a proof mass, the efficiency of the system can be greatly improved. For plucking speeds between 3 and 19 r/s, average output powers in the order of tens of microwatts were obtained for continuous plucking. By combining interaction energy, friction, and energy absorption, between the harvester and inertial mass, the maximum efficiency of the impact piezoelectric harvesters was found to be 1.4%. The efficiency results obtained were compared with the noncontact magnetic plucking approach further demonstrating the potential of our concept. Finally, different tip-gear materials combinations were evaluated showing the importance of their nature on the reliability of the presented configuration. [2014-0102]

Modal Parameter Tuning of an Axisymmetric Resonator via Mass Perturbation

Abstract: This paper reports the permanent frequency mismatch reduction of the primary wineglass modes in a planar axisymmetric resonator by strategic mass loading. The resonator consists of a set of concentric rings that are affixed to neighboring rings by a staggered system of spokes. The outer layers of spokes are targets for mass deposition. This paper develops modified ring equations that guide the mass perturbation process, and despite the fact that the deposited mass and deposition locations are quantized, it is possible to systematically reduce the frequency difference of the wineglass modes to effective degeneracy such that two modes cannot be distinguished in a frequency response plot. Results on five resonators are reported with nominal wineglass modes near 14 kHz, quality factors of 50k, and frequency mismatches exceeding 30 Hz in some cases, but with postperturbation mismatches smaller than 80 mHz. Furthermore, it is also shown that the quality factors remain unchanged. [2014-0227]

Thermoelastic Damping in the Size-Dependent Microplate Resonators Based on Modified Couple Stress Theory

Abstract: The dynamic properties and behaviors of microplate resonators have been experimentally shown to be size dependent. The thermoelastic damping plays an important role on the inherent energy dissipation of the microplate resonators. Based on the modified couple stress theory, the size-dependent thermoelastic damping in microplate resonators is investigated. The governing equation of motion is derived by using Hamilton principle. The thermoelastic damping is obtained via solving the heat diffusion equation. The presented results of thermoelastic frequencies have a good agreement with the reported values. The result shows that the size effect has significant impact on the thermoelastic damping when the plate thickness has a similar value to the material length scale parameter. It demonstrates that the thermoelastic damping can be suppressed and the quality factor can be enlarged as the material length scale parameter increases. The quality factor is improved by several orders of magnitude as the representative temperature drops from 500 to 80 K. However, the size-dependent quality factor at 400 K is larger than that at 293 K when the thickness of the plate has a similar value of the material length scale parameter. In addition, the differences among different plate materials are small, as the plate thickness is less than the characteristic thickness. However, those gaps become larger when the characteristic thickness is overtaken. [2013-0336]

In-Plane and Out-of-Plane MEMS Gyroscopes Based on Piezoresistive NEMS Detection

Abstract: This paper presents a new design and a complete characterization of amplitude-modulation gyroscopes based on piezoresistive nanogauges. The working principle and optimization criteria of in-plane and out-of-plane devices relying on double frame decoupling and levered sense mode are discussed in light of sensitivity and resolution theoretical predictions. The architecture of driving and sensing electronics is also presented. The reduced thermo-mechanical damping with respect to capacitive configurations, and the inherently high output signal leads to white noise performance in the mdps/ $\surd $ Hz range within an area smaller than 0.35 mm $^{\mathrm { {2}}}$ , at pressures in the millibar range. Sub-5-ppm linearity errors within 1000 dps are also demonstrated. [2015-0064]

Metallic Glass Hemispherical Shell Resonators

Abstract: By utilizing bulk metallic glasses' (BMGs) unique combination of amorphous structure, material properties, and fabrication opportunities, ultrasmooth and symmetric 3-D metallic glass resonators that are complimentary metal oxide semiconductor (CMOS) post-processing compatible are fabricated. Surface roughness to size ratio fabrication precision in the order of 100 parts per billion is demonstrated with a 3-mm diameter Pt 57.5 Cu 14.7 Ni 5.3 P 22.5 BMG hemispherical shell with a thickness variation <;100 nm and a surface roughness of <;1 nm Ra. The resonator exhibits a resonant frequency of 13.9440 kHz ± 0.1 Hz with 0.035% frequency mismatch between degenerate N = 2 wineglass modes with a quality factor of 6200. This performance was obtained in the asmolded state without any device tuning or trimming. Another resonator with N = 2 resonant modes at 9.393 and 9.401 kHz, and quality factors of 7800 and 6500 was mounted into an integrated electrode system. Electrical readout by capacitive sensing in both time and frequency domains showed a resonance shift to 9.461 and 9.483 kHz, respectively. The quality factor was reduced to 5400 and 5300, respectively. This investigation demonstrates that BMG resonators may serve as a basis for robust microelectromechanical systems resonator devices with increased performance and low-cost fabrication techniques that exploits the atomic structure, unique softening behavior, strength, formability, and toughness of metallic glasses.

Thin Flexible Thermal Ground Planes: Fabrication and Scaling Characterization

Abstract: Thermal ground planes (TGPs) are passive thermal management devices that utilize the latent heat associated with phase change to achieve high effective thermal conductance, similar to heat pipes. In this paper, we develop flexible TGPs with an ultra-thin thickness of 0.5 mm using copper-cladded polyimide as the encasing material, woven copper mesh as a wick, and electroplated copper pillars to support a vapor core. The lowest thermal resistance of one TGP is characterized to be only 1/3 that of an equivalently sized copper heat spreader. The effects of size scaling of evaporator and condenser, and overall TGP sizes on the thermal resistances of TGPs are experimentally characterized. A simple series thermal resistance model, which accounts for vapor core thermal resistance, is developed to predict the measured results. This experimentally validated model can be used for the design of TGPs with varying sizes of evaporator and condenser, and overall size. [2015-0032]

Characterization and Fatigue of the Converse Piezoelectric Effect in PZT Films for MEMS Applications

Abstract: A measurement setup for the detailed study of the transverse piezoelectric coefficient ${e} \mathbf {{}_{31,f}} $ in the converse (actuator) mode was developed. It allows the assessment of the piezoelectric stress in thin films on silicon cantilevers and provides for a correlation of this stress with large and small signal responses to ferroelectric polarization and dielectric response, both as a function of slowly sweeping electric field. This test is important for the understanding of piezoelectric thin films in microelectromechanical systems. The method is illustrated at hand of sol-gel lead-zirconate-titanate (PZT) thin films, and verified also with AlN and AlN-ScN alloy thin films. A 1- $\mu \text{m} $ thick, sol-gel derived PZT(53/47) gradient-free sample showed a response of −18.3 C/ $\text{m}\mathbf {^{2}} $ at 100-kV/cm electric field. Reliability tests of PZT thin films were carried out with the same setup in an accelerated manner. The piezoelectric activity did not degrade significantly up to 109 unipolar pulses at 100 kHz with an amplitude of −150 kV/cm. The increase in leakage toward the end of the cycles was explained by a thermal runaway effect. [2014-0140]

An LC -Type Passive Wireless Humidity Sensor System With Portable Telemetry Unit

Abstract: This paper presents a high-sensitivity passive wireless humidity sensor system with a portable telemetry unit for applications in sealed environments. A complementary metal oxide semiconductor (CMOS) interdigital capacitive humidity sensor die was attached to an organic substrate (FR-4) on which a fixed planar spiral copper inductor was fabricated. The variable capacitor and the fixed inductor were wire bonded to form an inductor–capacitor ( LC ) tank circuit. The resonant frequency of the sensor tank is dependent on the sensor capacitance, which changes in response to the humidity. The sensitivity of the capacitive sensor was improved significantly using graphene oxide as a sensing material. The package-level integration was achieved by employing the embedded inductor on an organic packaging substrate. The LC -type sensor is interrogated wirelessly using our homemade portable telemetry unit, which is based on a standing wave ratio bridge to measure the real part of the readout coil impedance. Measurements show a sensitivity of −18.75 kHz/%RH over a range of 15%–95% RH. The implemented telemetry unit addresses the need for a low-cost, portable, and universal reader of the LC -type passive wireless sensors. [2013-0188]